The present invention relates to motion-detection systems of the microwave variety. More particularly, it relates to a supervisory circuit for monitoring the transmission of the microwave generator component and for providing a signal in the event of a transmission failure.
Over the last several decades, microwave systems have proven useful in detecting the motion of objects in a region under surveillance. Operating on the well-known Doppler principle, such systems function by transmitting microwave radiation through a region in which the motion of objects is anticipated, and monitoring the radiation reflected by objects within such region for frequency shifts caused by the Doppler effect. Such systems have long been used for tracking the motion of inanimate objects, e.g., ships, airplanes and automobiles. More recently, however, they have been used to detect the movement and, hence, presence of animate objects, e.g., intruders and pedestrian traffic, in a region under surveillance. Though the invention may find utility in all microwave detection systems, it is especially useful in intruder detection systems where the system is required to operate unattended for long periods of time with extremely high reliability.
The microwave energy-generating element of motion-detecting microwave systems of the above type often comprises a so-called "Gunn" diode which, owing to its physical make-up, can be made to osciallate at a desired microwave frequency (e.g., 10.5 Gigahertz) in response to an applied voltage. The diode is positioned within a resonant cavity which is tuned to a desired resonant frequency. The cavity cooperates with a suitable antenna, e.g. a "horn" antenna, to direct the microwave energy toward the region under surveillance. A small portion of the microwave energy reflected by objects with such region is returned to the microwave cavity where it mixes with the transmitted energy. Depending on the net direction of movement of objects irradiated by the microwave energy, the reflected energy will be shifted, up or down, in frequency relative to the transmitted energy. This frequency shift (i.e., the Doppler frequency) can be detected by either of two techniques, i.e., the heterodyne technique, or the autodyne technique.
According to the heterodyne technique, a separate receiver diode is used to monitor the reflected microwave energy. The receiver diode is specially designed to detect microwave energy of the transmitted frequency. It is structurally similar to the Gunn diode, equally costly to fabricate, and significantly more prone to failure. In the heterodyne system, the receiver diode mixes the frequencies of the transmitted and reflected energy and generates sum and difference products, the difference product being the Doppler signal representing target motion. Compared to the autodyne technique, the heterodyne technique is the more sensitive; but, as already indicated, the more costly.
The less sensitive autodyne technique affords real advantages in terms of simplicity, reliability and cost. According to the autodyne technique, sometimes referred to as the "self-detect" technique, the Gunn diode provides a double-duty of both transmitting and receiving microwave energy. In response to reflected microwave energy returning to the cavity, the current flow in the Gunn diode is modulated at the Doppler frequency. To detect this Doppler frequency, an impedance is connected between the Gunn diode and ground, and the current through the impedance is monitored. By peak-detecting the current in this impedance, the Doppler signal is obtained directly. For many applications where high sensitivity is not required, the autodyne scheme is highly preferred. Such applications include those in which there is an abundance of detectable signal.
Since microwave radiation is invisible, the operability of the microwave energy-producing element (i.e., whether it is transmitting or radiating energy or not) is not discernible by mere observation. Thus, in intrusion detection and similar applications, where the microwave system is often required to 1 reliability and without frequent status checks, it is common to incorporate a supervisory circuit which functions to provide a "trouble" signal or alarm of some sort in the event of a transmission failure. Such a supervisory circuit is especially desirable in microwave systems where the Gunn diode, by its very nature, is relatively failure-prone. For example, Gunn diodes are known to simply stop oscillating in response to certain types of ambient temperature changes or relatively small changes in the applied bias voltage. Without such supervisory circuit, there is no easy way to detect a transmission failure.
In heterodyne microwave systems, the above-mentioned supervision can be achieved by positioning the receiver diode directly within the transmitting field of the Gunn diode. Such an arrangement is disclosed, for example, in the commonly assigned U.S. Pat. No. 4,660,024 to R. L. McMaster. In addition to providing a reference signal for subsequent Doppler frequency detection, the energy received directly from the Gunn diode serves to bias the receiver "on", thereby demonstrating to a supervisory circuit that the Gunn diode is indeed transmitting, and that the receiver diode is indeed receiving.
In the simpler autodyne systems, however, supervision of the operating status of the transmitter is not as direct as in the heterodyne system. In the autodyne system, supervision is usually achieved by detecting a disruption or sudden increase in the bias current in the aforementioned impedance used to develop the Doppler signal. The problem with this type of supervision is that this type of change in the current through the bias impedance occurs only in the event the Gunn diode either shorts out or opens up, i.e., in the event of a catastrophic failure. It does not verify that the Gunn diode is actually oscillating or transmitting energy. In fact, Gunn diodes often stop oscillating without either shorting out or opening. Should this occur, it would go undetected by the supervisory circuit.